1. Field of the Invention
The invention relates to a process for preparing polyvinyl alcohol by means of catalytic transesterification of an alcoholic polyvinyl ester solution and subsequent isolation of the polyvinyl alcohol formed.
2. Background Art
The preparation of polyvinyl alcohol (PVAL) by transesterifying polyvinyl esters obtained by free-radical polymerization of vinyl esters has long been known. Vinyl esters that can be used include esters of aliphatic carboxylic acids, such as vinyl acetate or vinyl propionate, for example. The transesterification usually takes place in the presence of monohydric aliphatic alcohols such as methanol or ethanol. The reaction may take place with either basic or acidic catalysis. The polyvinyl alcohols obtained are described primarily by their degree of hydrolysis and their viscosity in a 4% by weight aqueous solution.
Another characteristic frequently encountered instead of the degree of hydrolysis is the saponification number, which indicates the amount of KOH in milligrams per gram of PVAL that is needed for complete cleavage of all of the remaining ester groups. The saponification number indicates only the mean value of the KOH consumption for complete ester cleavage. For identical measured saponification numbers, therefore, it is possible for distinctly different breadths of the saponification number distribution to occur. Both the breadth of the saponification number distribution and blockiness have a great influence on the quality of the PVAL, especially for its use as a protective colloid in emulsion and suspension polymerization.
For identical saponification number and viscosity, there are nevertheless distinct differences in the makeup of polyvinyl alcohols depending on the preparation process. As an initial consideration, the esters groups remaining may be distributed differently, for example concentrated in relatively large blocks or distributed randomly. When basic catalysts are used, the polyvinyl alcohols obtained tend to be blocky; when using acidic catalysts, polyvinyl alcohols with random distribution of remaining ester groups are more likely to be obtained. Additions of other solvents, such as water, also influence the distribution of the ester groups in the polyvinyl alcohol.
One established process for preparing polyvinyl alcohol is known as the belt process, and is described, for example, in U.S. Pat. No. 3,278,505 and DE-A 2251603. In this process, aqueous alkali is mixed rapidly and thoroughly with generally methanolic polyvinyl acetate solution and the mixture is applied to a continuous belt. The mixture solidifies to a gel, which at the end of the belt is fractionated and cut. The resulting granules are generally neutralized with acetic acid and washed with methanol. This continuous process is especially suitable for preparing polyvinyl alcohol in large amounts, an advantage being very economical preparation of large amounts of polyvinyl alcohol with uniform product quality. A disadvantage is that the granulated gel is difficult to neutralize; associated with this difficulty is an undesirable increase in the breadth of the saponification number distribution. The process is economical only for the production of large amounts of PVAL. It is therefore prohibitive to employ for the introduction of new and innovative products in quantities which, at commencement of production, are small. Further disadvantages include the high acquisition costs and the large amount of space occupied by the plant.
A process very similar to the belt process is the extruder process, which is described, for example, in EP-B 54716 and EP-A 942008, in which the belt is replaced by an appropriate extruder. In this process, the gel is comminuted in the extruder during the reaction. In comparison to the belt process, the extruder process can be used to process more highly concentrated polyvinyl acetate solutions. A disadvantage are the acquisition costs, which are even higher than those for the belt process, at identical capacities.
Another continuous process which has been described is the suspension transesterification of polyvinyl acetate, U.S. Pat. No. 3,487,060, for example. Here, the polyvinyl acetate solution and the catalyst solution are added continuously to an agitated alcoholysis mixture, and a slurry of polyvinyl alcohol in methanol and methyl acetate is removed continuously from this alcoholysis mixture. By means of an appropriate process regime, gel formation can be substantially prevented. In comparison to the other continuous processes, the acquisition costs are relatively low. However, it is difficult to maintain a specific degree of hydrolysis, and thus product uniformity suffers as a result.
Among the principal established noncontinuous processes is that of transesterification in a kneading apparatus, which is described, for example, in DE-A 3000750. A highly concentrated, generally methanolic, polyvinyl acetate solution is mixed with the transesterification catalyst. The resulting gel is progressively comminuted during the reaction. Following termination of the reaction with acid, methanol and methyl acetate formed in the kneading apparatus are separated by distillation. off. The kneading apparatus is very well suited to producing small quantities of polyvinyl alcohol specialties. Large amounts of a product are difficult to produce at favorable cost, however. Since the kneading apparatus is a poor mixer, both for the mixing in of the catalyst and for the addition of acid upon neutralization, the resultant polyvinyl alcohols have very broad saponification number distributions.
The transesterification of polyvinyl acetate to polyvinyl alcohol may also take place in a standard stirred tank, as is described, for example, in DE-A 2304684. Dilute, generally methanolic, polyvinyl acetate solutions are mixed with the catalyst. By progressive stirring, the gel is comminuted and a very fine suspension of polyvinyl alcohol in methanol/methyl acetate is obtained. As a result of the rapid distribution of the catalyst in the alcoholic solution and the uniform termination of the reaction with an acid in the fine suspension, it is possible to obtain polyvinyl alcohols of very good quality with a narrow saponification number distribution. Besides the solid polyvinyl alcohol obtained as a result of separating the solvent mixture, it is possible to distill off methyl acetate/methanol and at the same time to add water, in order to obtain an aqueous polyvinyl alcohol solution. A disadvantage is that owing to the poor filterability of the fine suspension, the preparation of solid polyvinyl alcohol by this process is not economical. Furthermore, large amounts of solvent must be distilled from the stirred tank and subsequently worked up by distillation. The twofold distillation represents a time-consuming and energy-intensive process. For separating methyl acetate and methanol, moreover, a plurality of distillation columns are required.
From DD-A 251683 it is known that the production of the aqueous solution may be accelerated by isolating the polyvinyl alcohol from the reaction mixture and introducing it, at a residual moisture content of more than 50% by weight, into a water-filled dissolution tank. If vacuum is applied, superheated steam introduced, and at the same time the dissolution tank is heated, methanol, methyl acetate, and water are distilled away and the polyvinyl alcohol goes into solution. This still leaves the problem of the poor filterability of the polyvinyl alcohol suspension. Another disadvantage of this process is that there are large quantities of filtrate to be reprocessed.
It is known that the filterability of the resulting polyvinyl alcohol suspension may be improved significantly by the addition of aliphatic or cycloaliphatic hydrocarbons. For example, DD-A 238054 describes producing a heterogenous methanolic polyvinyl acetate solution by adding 20-50% of hydrocarbons with a chain length of from 5 to 10 carbon atoms. Alkali-catalyzed transesterification conducted in this mixture leads to finely particulate, but readily filterable polyvinyl alcohol suspensions. The partially saponified polyvinyl alcohols obtained exhibit particularly good properties as protective colloids. A disadvantage of the process is that the resulting mixtures of hydrocarbon, methanol, and methyl acetate formed are very difficult to reprocess.
It is an object of the invention to develop a process for preparing polyvinyl alcohol wherein the filterability of the resulting polyvinyl alcohol suspension is improved. A further object of the invention is to develop a process which does not entail large quantities of solvent requiring distillative workup. The invention thus provides a process for preparing polyvinyl alcohol by means of catalytic transesterification of an alcoholic polyvinyl ester solution and subsequent workup by means of isolation of the polyvinyl alcohol formed, the process comprising adding the acetic ester of the alcohol used as solvent in an amount of from 5 to 70% by weight, based on the overall amount of alcoholic solvent and its acetic ester prior to transesterfication of the polyvinyl ester solution.
In a preferred embodiment, the polyvinyl alcohol formed in the transesterification is isolated from the liquid phase and the liquid phase is recycled to the transesterification in amounts such that the fraction of the acetic ester of the alcohol used as solvent (termed xe2x80x9cacetic esterxe2x80x9d herein) in the alcohol/acetic ester mixture remains within the range of 5 to 70 weight percent during the transesterification.
Suitable polyvinyl esters are the homopolymers and copolymers with vinyl esters of aliphatic carboxylic acids having from 1 to 12 carbon atoms, such as vinyl formate, vinyl acetate, vinyl propionate, and also vinyl esters of alpha-branched carboxylic acids having from 9 to 10 carbon atoms (VeoVa 9 or VeoVa 10, trade names of Shell). If desired, the polymers may also include up to 50% by weight of comonomer units which derive from further, ethylenically unsaturated comonomers which are copolymerizable with vinyl esters. Examples include olefins such as ethylene and propylene, and also ethylenically unsaturated monocarboxylic and dicarboxylic acids such as acrylic acid or methacrylic acid, ethylenically unsaturated carboxamides such as acrylamide and N-methylolacrylamide, and isopropenyl acetate. The most preferred polyvinyl ester is polyvinyl acetate.
Suitable alcoholic solvents for the polyvinyl esters are monohydric aliphatic alcohols having from 1 to 4 carbon atoms or mixtures thereof, preferably methanol or ethanol, with particular preference being given to methanol. The amount of polyvinyl ester in the solution is from 20 to 80% by weight, preferably from 30 to 70% by weight. Before the onset of transesterification, acetic ester and, where appropriate, alcohol, are added to the polyvinyl ester solution in an amount such that the amount of acetic acid is from 5 to 70% by weight, preferably from 15 to 40% by weight, most preferably from 25 to 35% by weight, based in each case on the overall amount of alcoholic solvent and acetic ester. The amount of polyvinyl ester relative to the overall amount of the reaction mixture is from 2 to 75% by weight, preferably from 5 to 60% by weight. If methanol is used as solvent, methyl acetate is added as the acetic ester; in the case of ethanol as the solvent, ethyl acetate is added as the acetic ester. The addition of acetic ester takes place preferably in the form of an alcohol/acetic ester mixture which has been obtained as a filtrate during the workup of a preceding batch when batchwise operation is employed or isolated during the workup of the end product (polyvinyl alcohol) in continuous process.
The transesterification is initiated using customary acidic or alkaline catalysts. Examples of acidic catalysts are strong mineral acids such as hydrochloric acid or sulfuric acid, or strong organic acids, such as aliphatic or aromatic sulfonic acids. It is preferred to use alkaline catalysts. Examples of these catalysts are the hydroxides, alkoxides, and carbonates of alkali metals or alkaline earth metals. Preference is given to the hydroxides of lithium, sodium, and potassium; sodium hydroxide is particularly preferred. The alkaline catalysts are used in the form of their aqueous or alcoholic solutions, preferably in alcoholic solution, and with particular preference in the same alcohol that is used to dissolve the polyvinyl ester. The amounts of alkaline catalyst used are generally from 0.2 to 20.0 mol %, based on polyvinyl ester.
The transesterification is generally conducted at temperatures from 20xc2x0 C. to 60xc2x0 C., preferably from 30xc2x0 C. to 40xc2x0 C. In batchwise operation, the alcoholic polyvinyl ester solution is supplied to a reaction vessel, generally a stirred tank or a kneading apparatus, and acetic ester is added in an amount such as to give the abovementioned proportions of alcohol to acetic ester. To this end, a preferred procedure is to use the filtrate from preceding batches and to add, where appropriate, further alcohol or acetic ester in order to establish the desired proportion. Where appropriate, the batch may also be overlaid with filtrate or alcohol, taking into account said proportions of alcohol to acetic ester in the reaction vessel.
The transesterification is initiated by adding the catalyst solution. When the desired degree of hydrolysis, generally between 70 and 100 mol %, has been reached, the transesterification is terminated. In the case of acid-catalyzed transesterification, termination is effected by adding alkaline reagents. Examples of these are the hydroxides, alkoxides, and carbonates of alkali metals or alkaline earth metals. Preference is given to the hydroxides of lithium, sodium, and potassium; sodium hydroxide is particularly preferred. In the case of the preferred alkali-catalyzed tranesterification, termination is effected by adding acidic reagents, such as carboxylic acids or mineral acids. Preference is given to relatively strong carboxylic acids and mineral acids, preferably having a pKa of less then 4.5, with particular preference a pKa of less then 2.5. Examples of suitable mineral acids are hydrochloric acid, sulfuric acid, and nitric acid; examples of suitable carboxylic acids are oxalic acid, formic acid, aromatic and aliphatic sulfonic acids, and halocarboxylic acids, such as mono-, di,- or trichloroacetic acid.
After the end of the transesterification reaction, the polyvinyl alcohol formed during the transesterification is isolated from the liquid phase. This may be done by means of customary apparatus for solid/liquid separation, such as by centrifuging or filtration, for example. The resultant filtrate is preferably recycled to the reaction vessel for producing the subsequent batches. The recycle rate depends on the alcohol/acetic ester ratio used. Generally speaking, from 10 to 90% by weight, preferably from 50 to 80% by weight, of the filtrate is reused in subsequent batches.
The polyvinyl alcohol filtercake may be worked up conventionally, for example by drying and removing residual fractions of the alcohol/acetic ester mixture during the drying operation. The polyvinyl alcohol filtercake may also be taken up in water, usually with the production of an aqueous solution with a polyvinyl alcohol fraction of from 5 to 80% by weight, preferably from 10 to 50% by weight, with particular preference from 10 to 30% by weight, followed by subsequent vacuum treatment and/or stripping of the solution by passing inert gases such as steam over or through the solution, in order to remove residual fractions of alcohol and acetic ester.
In the case of a continuous operation, generally on a belt, or in a kneading apparatus or extruder, the alcoholic polyvinyl ester solution and the acetic ester are added in an amount such as to result in the abovementioned proportions of alcohol to acetic ester. For the addition of the acetic ester, it is preferred to add the alcohol/acetic ester mixture isolated in the workup of the end product, and, where appropriate, further alcohol or acetic ester in order to establish the desired proportion. The addition of the catalyst and the termination of reaction takes place in analogously to the batchwise operation. After the end of the reaction, the polyvinyl alcohol is isolated from the liquid phase. Isolation may be performed by the same procedure as described for batchwise operation, or by means of distillation. The resultant alcohol/acetic ester mixture may be recycled in the amounts specified for batchwise operation.
Owing to the addition of acetic ester before the beginning of the transesterification, the products obtained lead to polyvinyl alcohol suspensions that are more readily filtered. The principal advantage of the process is that, with transesterification in an alcohol/acetic ester mixture, the alcohol/acetic ester mixture obtained in the work up of the polyvinyl alcohol may be used again as the reaction medium, thereby avoiding the need to work up large amounts of solvent. Additionally, the preferred use of strong acids for the neutralization makes it possible to prevent the formation of buffer systems within the isolated alcohol/acetic ester mixture. When the solvent mixture separated off is to be used again as the reaction medium as in the present process, creation of a buffer system would result in increased alkali consumption. As a result, the polyvinyl alcohol obtained could contain an increased amount of residual salts, which frequently have disruptive effects on the end uses of the polyvinyl alcohol, for example as a protective colloid in emulsion and suspension polymerization.
The examples which follow serve to illustrate the invention.